It was used in Middle English as an adjective meaning "leaping, skipping." The meaning "pointing outward" (preserved in military usage) is from 1680s; that of "prominent, striking" first recorded 1840, from salient point (1670s), which refers to the heart of an embryo, which seems to leap, and translates Latin punctum saliens, going back to Aristotle's writings. Hence, the "starting point" of anything.